Many audio systems implement automatic level control (ALC) to dynamically adjust the signal level of an input signal, such as from a microphone. Generally speaking, ALC involves increasing the signal level (gain) of the input signal when the input signal level is below a predetermined minimum level and reducing the signal level (gain) of the input signal when the input signal level is above a predetermined maximum level, for example, using a programmable gain amplifier (PGA). Specifically, when the input signal level becomes smaller than a predetermined target level, the ALC will ramp up the gain of the PGA after some hold time. The gain change rate is referred to as the decay time. When the input signal level becomes above the target level, the ALC will reduce the gain of the PGA at a rate referred to as the attack time.
FIG. 1 is a schematic diagram providing an example of ALC operation, specifically showing a representation of an input signal 110, a representation of the PGA gain curve 120 performed by the ALC, and a representation of the output signal 130 resulting from ALC. In this example, the input signal 110 has a first portion 111 in which the signal level is within the target level, a second portion 112 in which the signal level is smaller than the target level, and a third portion 113 in which the signal level is above the target level. During the first portion 111 of the input signal 110, the ALC performs no PGA gain change, such that the signal level of a corresponding portion 131 of the output signal 130 is unchanged from the first portion 111 of the input signal 110. During the second portion 112 of the input signal 110, the input signal level becomes smaller than the target level. After a predetermined hold time 132, since the input signal level is still smaller than the target level, the ALC increases the gain of the PGA over a predetermined decay time 133 so that the output signal level reaches the target level and remains at the target level through time period 134, representing the remainder of the second portion 112 of the input signal 110. During the third portion 113 of the input signal, the input signal level becomes above the target level. The ALC decreases the gain of the PGA over a predetermined attach time 135 so that the output signal level reaches the target level and remains at the target level for time period 136.
The ALC sometimes provides a noise gate mode to deal with the situation in which there is little or no input signal level (e.g., when nobody is speaking into the microphone or the microphone is muted). When the signal is very quiet and consists mainly of noise, the ALC function may cause a phenomenon often referred to as “noise pumping.” The noise gate mode prevents noise pumping by comparing the signal level at the input against a noise gate threshold and controlling the gain of the PGA or other output control accordingly, such as, for example, setting the gain to zero, muting the output signal, or keeping the gain the same as it was before the signal was recognized as noise.
FIG. 2 is a schematic diagram providing an example of a noise gate mode, specifically showing a representation of an input signal 210, and a representation of the PGA gain curve 230, and a representation of the output signal 220 resulting from the ALC with noise gate mode. In this example, the input signal 210 has a first portion 211 in which the signal level is above the noise gate threshold, a second portion 212 in which the signal level is below than the noise gate threshold, and a third portion 213 in which the signal level is above the noise gate threshold. During the first portion 211 of the input signal 210, the ALC increases the gain of the PGA 230 so as to increase the output signal level 220 into the target level. During the second portion 212 of the input signal 210, the ALC holds the gain level of the PGA 230 so that the noise is not amplified in the output signal 220. During the third portion 213, the ALC increases the gain of the PGA 230 so that the output signal level 220 is at the target level.
Thus, the ALC can boost low level signals to make them heard more clearly, limit high level signals at a fixed level to avoid output clipping, and eliminate noisy signals from the output or keep noisy signals at a very low level in the output. ALC is usually integrated into post processing chips that process raw microphone data from the microphone(s).
In microphone array systems such as MEMS microphone arrays, some amount of signal processing often is incorporated into each of a plurality of microphone devices for local processing the microphone input signal by the microphone device. As described in U.S. patent application Ser. No. 13/426,918, data may be passed from one microphone device to another microphone device in a daisy-chain configuration to allow for serialized signal processing, such as for beamforming, noise reduction/cancellation, or acoustic source localization, to name but a few.